High reliability second level interconnects using polymer core BGAs

The growing need for shorter interconnects for better electrical performance has shifted package designs more and more towards ball grid arrays (PGAs). This has brought into focus the requirement for a reliable second level interconnect. Due to the thermal mismatch between the package material (e.g. ceramic) and the PWB board the solder interconnects are prone to failure during temperature excursions. Increasing the interconnect height by using a larger diameter ball may increase the mechanical reliability, but would compromise the electrical performance. So there needs to be an interconnect solution to getting good reliability without compromising the electrical properties. This paper compares the mechanical reliability and electrical performance of a novel interconnect material with the conventional solder ball. The new solder ball consists of a core of polymer with a thin layer of copper with additional covering of solder which could be either tin/lead based or lead free. Temperature cycling data was obtained from both conventional and polymer core balls tested on ceramic packages mounted on a FR4 board. Weibull data showing the differences are plotted and failure analysis is done on the failed parts to understand the different failure modes between both the types of BGAs. An electrical simulation was done to compare the polymer-core ball with a conventional solder ball. DC resistance analysis and a full-wave electromagnetic RF simulation were performed to compare the two types.